Electromagnetic wave attenuating material has commonly been placed on ships, planes, and other vehicles, particularly those used in the military, for several reasons. The radar system used on a ship generates radar signals. These radar signals may reflect off the ship's own structure, creating false echoes or ghost images. These false echoes may interfere with the proper navigation of the ship, for example, because of increased clutter on radar. These false echoes may also be represented as false targets on the radar. Electromagnetic wave attenuating material (also known as radar absorbing material) can be bonded to selective areas of the ship where reflection of the ship's radar signals commonly occurs. The electromagnetic wave attenuating material attenuates the radar signals, preventing back reflection.
Another application of electromagnetic wave attenuating material involves antenna radiation pattern shaping. The presence of conductive objects near an antenna can alter the established free space propagation characteristics of the antenna. However, placing electromagnetic wave attenuating material in those conductive areas near the antenna eliminates the problem. The use of electromagnetic wave attenuating material on ships, planes, and other vehicles also reduces the vehicle's cross-sectional area as seen by the radar. Reducing the radar cross section reduces the vehicle's signature, that is, its ability to be detected by radar.
Attenuation of electromagnetic waves is represented by the following equation: EQU Attenuation (dB)=20 log A/A.sub.0
4 where A.sub.0 is the original signal amplitude and A is the signal amplitude after passing through the electromagnetic wave attenuating material. Attenuation generally occurs as a result of two mechanisms. One mechanism involves destructive interference between a first wave and a reflected second wave which is 180.degree. out of phase with the first wave. Another attenuation mechanism occurs by the absorption of the electromagnetic wave energy.
With regard to the absorption of the electromagnetic wave energy, the energy of an electromagnetic wave is a function of the distance it travels through a medium, as represented by the following equation: EQU E(x)=E.sub.o e.sup.-2x/.delta.
where E.sub.o is the original energy, E(x) is the remaining energy, x is the distance traveled in the medium and .delta. is the "skin depth." For a good conductor, .delta. is proportional to: ##EQU1## where .mu. is the magnetic permeability and .sigma. is the conductivity. Magnetic materials such as ferrites, iron and cobalt-nickel alloys are used to alter the permeabilities of materials. For example, the magnetic materials can be embedded in a rubber or elastomer. Increasing a material's magnetic permeability value increases the material's absorption of electromagnetic wave energy.
To achieve destructive interference between electromagnetic waves, typically a thin material is provided that is effectively one-fourth wavelength of the electromagnetic wave energy wavelength incident upon the material. The electromagnetic waves incident upon this material will be reflected or transmitted depending upon the properties of the material, as represented by the following equation: ##EQU2## where: Z.sub.o =.mu..sub.o /e.sub.o
Z.sub.l =.mu..sub.l /e.sub.l
and where .mu. is the magnetic permeability and e is the electric permittivity. Increasing the permeability of a material, such as by embedding magnetic particles in an elastomer, will cause electromagnetic waves to be partially reflected off of this material and partially transmitted through this material if the electromagnetic wave was traveling through a medium having a lower Z.sub.l (for example, if the permeability of the medium is lower than the permeability of the material). To achieve destructive cancellation of electromagnetic waves, a one-fourth wavelength material is used which has a higher permeability than the medium (typically the medium is air) through which the electromagnetic wave is traveling. This one-fourth wavelength material causes a reflected and transmitted wave. In practice, the one-fourth wavelength material is placed on a conductive backing so that the electromagnetic waves transmitted through the material will reflect off of the conductive backing. The transmitted electromagnetic wave, upon reflection from the conductive backing, will emerge 180.degree. out of phase with the electromagnetic wave reflected by the one-fourth wavelength material. These reflected and emerging waves destructively interfere with each other, thereby resulting in cancellation.
One type of structure commonly used to cause electromagnetic wave attenuation is known as a dual band absorber. Dual band absorbers are generally made of two layers bonded together. The top layer, which may be loaded with magnetic particles, is typically an elastomer having a slightly higher permeability than free space, while the bottom layer is an elastomer having a higher concentration of magnetic particles embedded in it to increase the permeability above the value of the top layer. The dual band absorber is placed on a conductive backing. The top and bottom layers will also have electromagnetic wave energy absorbing properties, as noted above. These dual band absorbers, because of their composition, typically provide peak attenuation (greater than 20 dB) of the electromagnetic waves at two specific frequencies, while broadband absorption of 10-15 dB is generally obtained between the two null frequencies.
The electromagnetic wave attenuation performance of these materials, such as the dual band absorber, is adversely affected by accumulations of ice which can form thereon. Ice formation can also severely inhibit vehicle performance, particularly if it is an air vehicle. Heating the surface of the electromagnetic wave attenuating structure to remove ice would require placing a heat source underneath the electromagnetic wave attenuating layers so that the heat source would not interfere with the attenuation properties. Therefore, higher power is required from the heat source to provide enough thermal energy to deice the surface. Furthermore, the higher temperatures produced could be detrimental to the materials used in the electromagnetic wave attenuator. Heating sources such as electrothermal devices are effective at deicing, however, such devices are generally made of highly conductive materials which interfere with the attenuation performance of the electromagnetic wave attenuation layers.
One type of commercially available deicer which does not employ heat and which is not constructed with highly conductive materials is a pneumatic deicer, such as disclosed in Kageorge U.S. Pat. No. 4,687,159. These pneumatic deicers have a deformable sheet secured to the surface of the vehicle where ice accumulation is to be prevented. The deicer also has spaced, parallel, inextensible threads to define a series of inflatable sections in the deicer which may be alternately expanded and contracted by fluid pressure to break up ice accumulation. The pneumatic deicer does not, however, have electromagnetic wave attenuating properties.
Therefore, it is an object of this invention to provide a structure which, when placed on the surface of a vehicle, will attenuate electromagnetic waves and will eliminate ice accumulation on the surface of the structure so that ice will not interfere with the electromagnetic wave attenuation performance. It is a further object to provide a structure which can prevent ice accumulation and which attenuates electromagnetic waves over a particular frequency range. Another object is to provide an electromagnetic wave attenuating structure which can prevent ice accumulation and which has an outer layer which is weather and wear resistant.